This invention relates generally to charge transfer devices and more particularly to input circuitry for such devices.
As is known in the art, radar system signal processors are used for determining the Doppler frequency associated with detected moving objects. In such radar system a bipolar video signal, or range sweep, is produced in response to each one of a train of pulses. The pulses are transmitted at the radar system pulse repetition frequency (PRF), typically in the order of several KHz. At predetermined times after each one of the pulses in the train is transmitted the bipolar video signal, or range sweep, is sampled. Each one of the samples corresponds to a radar return from an object at a corresponding range or range cell. The rate of change in sample level associated with any one range from range sweep to range sweep is indicative of the Doppler frequency of an object at such range cell. As is also known, the Doppler frequency of such object may be determined by passing the range sweep to range sweep samples associated with such object through a frequency spectrum analyzer. Signal processors of the type described have been implemented using various digital processing techniques; however, such processors generally require that the samples be converted into corresponding digital words, as with analog-to-digital converters, and then processed by suitable logic circuitry, thereby adding cost to the signal processor.
One technique which has been suggested to reduce the cost of such a radar system signal processor includes the use of a plurality of charge transfer devices. Each one of the charge transfer devices is associated with the samples of a corresponding one of a plurality of range cells. The samples produced from a plurality of range sweeps are stored in corresponding ones of the plurality of charge transfer devices. That is, the samples in each range sweep are stored, sequentially, into a corresponding one of the plurality of charge transfer devices and once stored are shifted from stage to stage each time a radar pulse is transmitted. That is, the stored samples are transferred from stage to stage at the radar system pulse repetition frequency (PRF), typically in the order of several KHz, as noted above. Once a predetermined number of samples is stored, such number being selected in accordance with the desired Doppler frequency resolution, the samples are passed for analysis to a frequency spectrum analyzer. With such arrangement it is noted the samples of a range sweep are taken in a sampling interval which is significantly shorter than the radar system pulse repetition interval (PRI=1/PRF). That is, the sampling interval, or window, is typically in the order of several nanoseconds, while the PRI is in the order of several microseconds.
Two types of charge transfer device input arrangements are sometimes referred to as the "gated input" technique and the "potential equilibration" technique. Such techniques are described in a book entitled Charge Transfer Devices by Carlo H. Seguin and Michael F. Tompsett, published by Academic Press, Inc., New York, N.Y., 1975. With the "gated input" technique the potential energy level of the input (or source) diffusion region is controlled by the input signal being sampled (i.e. here the range sweep signal). Just prior to the time a sample of such input signal is to be taken a voltage pulse is applied to an adjacent sampling gate to lower the potential of a "gate" region in the device which is juxtaposed the source diffusion region and thereby allows an amount of charge related to the level of the input signal to pass from the source diffusion region through the gate region to an input region which is juxtaposed the gate region. The level of the charge passed to the gate region is related to the level of the input signal; however, such relationship is nonlinear because the depletion capacitance at the surface of the device is related to the surface potential and hence is related also to the input signal level. Further, such "gated input" technique is relatively noisy because of fluctuations in the partitioning of the charge stored temporarily in the "gate" region during the sampling process.
The potential equilibration technique is used with a charge transfer device which has a source diffusion region, first input gate region and second input gate region disposed, successively, along the charge transfer device. The input signal being sampled is fed to the second input gate region. A reference voltage is fed to the first input gate region to maintain the level of potential energy at the surface of such region at a level greater than the maximum expected level of potential energy at the surface of the second input gate region. The source diffusion region is fed by a pulsed voltage source. Charge is injected by the source diffusion and is shifted into the device at a predetermined clock rate. During each clock interval the potential energy at the source diffusion region is first raised so that the first and second input gate regions are filled with a charge, then lowered so that the charge remaining in the second input gate region is proportional to the difference in voltage between the reference voltage level and the input signal level. The charge remaining in the second input gate region is then shifted to a first storage stage region of the device. During the next clock interval the process repeats. The charge in the first storage stage region is transferred to the next storage stage region during the first half of such clock interval and a new charge is stored in the first storage stage during the second half of such clock interval. The sampling interval, or window, may therefore be considered as the time interval between the time at which the potential energy level of input diffusion region is lowered and the time at which the charge is stored in the first storage stage region. Because the equilibration in charge in the first and second input gate regions always occurs when the surface of the first input region is always at the same potential level, i.e. the reference voltage, the charge stored in the second input gate region is, after equilibration, linearly proportional to the sampled level of the input signal. It should be noted, however, a sampling characteristic of such equilibration technique is that any decrease in input signal level (i.e. rise in potential) between the time at which the input diffusion region potential is lowered and the time the equilibrated charge is stored in the first storage stage region will reduce the amount of actual charge stored in such first input storage stage, the lowest level of such input signal in such time interval thereby being stored in the charge transfer device. In the radar system signal processor application discussed above the clock period would be related to the radar system pulse repetition interval (RPI). In such a processor a plurality of charge transfer devices is included, each one of such devices being used to store samples of a corresponding one of a plurality of range cells. With the equilibration technique described above each device would generally require individual clock pulses and diffusion region voltage pulses, or a buffer stage, to provide the requisite range cell sampling, thereby increasing the cost and complexity of the processor. This is particularly the case when it is desired to form the plurality of charge transfer devices in a single crystal body.
As is also known in the art, a radar system signal processor which employs the use of a plurality of charge transfer devices for storing samples of radar returns, each one of the devices storing samples associated with a corresponding one of a plurality of range cells, also includes an input charge transfer device to distribute the samples of each range sweep into corresponding ones of the plurality of charge storage devices. Where it is desired to form all the charge transfer devices as a single crystal body the input charge transfer device is disposed along a vertical direction and the plurality of range cell storage charge transfer devices is disposed along a horizontal direction, each one of the stages of the input charge transfer device being coupled to a corresponding one of the plurality of charge transfer devices. However, in a practical charge transfer device the width of each device is in the order of 5 mils. Because charge transfer devices generally require very short channel lengths (typically less than 0.4 mils) for high transfer efficiency, with such arrangement a compromise must be made in the dynamic range of the plurality of horizontally disposed devices or the transfer efficiency of the vertically disposed device or both.